feat: Generate HotReload-friendly merged file when HR is enabled

src/Directory.Build.props

@@ -2,6 +2,7 @@
 
 	<PropertyGroup>
     <Authors>unoplatform</Authors>
+	<LangVersion>11</LangVersion>
     <PackageProjectUrl>https://github.com/unoplatform/uno.xamlmerge.task</PackageProjectUrl>
     <PackageIconUrl>https://nv-assets.azurewebsites.net/logos/uno.png</PackageIconUrl>
     <RepositoryUrl>https://github.com/unoplatform/uno.xamlmerge.task</RepositoryUrl>

src/Uno.XamlMerge.Task/BatchMergeXaml.cs

@@ -11,6 +11,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Text;
 using System.Xml;
 
 namespace Uno.UI.Tasks.BatchMerge
@@ -28,6 +29,9 @@ public class BatchMergeXaml_v0 : CustomTask
         [Required]
         public string ProjectFullPath { get; set; }
 
+        [Required]
+        public bool IsHotReloadEnabled { get; set; }
+
         [Output]
         public string[] FilesWritten
             => _filesWritten.ToArray();
@@ -44,6 +48,12 @@ public override bool Execute()
             var filteredPages = Pages.ToList();
             filteredPages.RemoveAll(e => MergedXamlFiles.Any(m => FullPathComparer.Default.Equals(e, m)));
 
+            if (IsHotReloadEnabled)
+            {
+                GenerateForHotReload(filteredPages);
+                return !HasLoggedErrors;
+            }
+
             if (MergedXamlFiles.Length > 1)
             {
                 foreach (var mergedXamlFile in MergedXamlFiles)
@@ -69,6 +79,70 @@ public override bool Execute()
             return !HasLoggedErrors;
         }
 
+        private string GenerateMergedDictionariesForHotReload(IEnumerable<ITaskItem> filteredPages)
+        {
+            var projectBasePath = Path.GetDirectoryName(Path.GetFullPath(ProjectFullPath));
+
+            var builder = new StringBuilder();
+            builder.Append("""
+                        <!-- Generating a resource dictionary that references existing dictionaries for HotReload support -->
+                        <!-- Proper merging of XAML files will happen if you build in Release configuration -->
+                        <ResourceDictionary
+                            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
+                            <ResourceDictionary.MergedDictionaries>
+
+                        """);
+
+            foreach (var page in filteredPages)
+            {
+                var pagePath = Path.GetFullPath(page.ItemSpec).Replace(projectBasePath, "").TrimStart(Path.DirectorySeparatorChar);
+                builder.Append($"""
+                                    <ResourceDictionary Source="ms-appx:///{pagePath}" />
+                            """);
+            }
+
+            builder.Append("""
+                            </ResourceDictionary.MergedDictionaries>
+                        </ResourceDictionary>
+
+                        """);
+
+            return builder.ToString();
+        }
+
+        /// <summary>
+        /// When HotReload is enabled, we want modifications to the original XAML files to be reflected.
+        /// This cannot be achieved when an actual merge happens.
+        /// So, for HotReload, we'll generate a XAML file that only references the original XAML files and not do an actual merge.
+        /// This way, it can work with HotReload perfectly.
+        /// </summary>
+        private void GenerateForHotReload(List<ITaskItem> filteredPages)
+        {
+            if (MergedXamlFiles.Length > 1)
+            {
+                foreach (var mergedXamlFile in MergedXamlFiles)
+                {
+                    var mergeFileName = Path.GetFileName(mergedXamlFile.ItemSpec);
+                    string fileContents = GenerateMergedDictionariesForHotReload(filteredPages.Where(p => string.Equals(p.GetMetadata("MergeFile"), mergeFileName, StringComparison.OrdinalIgnoreCase)));
+
+                    Directory.CreateDirectory(Path.GetDirectoryName(mergedXamlFile.ItemSpec));
+                    Utils.RewriteFileIfNecessary(mergedXamlFile.ItemSpec, fileContents);
+                }
+            }
+            else if (MergedXamlFiles.Length == 1)
+            {
+                // Single target file, without "MergeFile" attribution
+                var mergedXamlFile = MergedXamlFiles[0];
+                var mergeFileName = Path.GetFileName(mergedXamlFile.ItemSpec);
+                string fileContents = GenerateMergedDictionariesForHotReload(filteredPages);
+
+                Directory.CreateDirectory(Path.GetDirectoryName(mergedXamlFile.ItemSpec));
+                Utils.RewriteFileIfNecessary(mergedXamlFile.ItemSpec, fileContents);
+            }
+        }
+
         private void ValidatePageMergeFileMetadata()
         {
             if (MergedXamlFiles.Length > 1)

src/Uno.XamlMerge.Task/Uno.XamlMerge.Task.csproj

@@ -4,7 +4,6 @@
 		<TargetFramework>netstandard2.0</TargetFramework>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>
-		<LangVersion>10.0</LangVersion>
 		<TreatWarningsAsErrors>True</TreatWarningsAsErrors>
 		<IsTool>true</IsTool>
 		<AssemblyName>Uno.XamlMerge.Task.v0</AssemblyName>

src/Uno.XamlMerge.Task/build/Uno.XamlMerge.Task.targets

@@ -12,9 +12,11 @@
 			DependsOnTargets="_XamlMergeFillProperties"
 			Condition="'$(BuildingProject)' == 'true' or '$(DesignTimeBuild)' != 'true'">
 
+    <!-- TODO: Is Configuration==Debug the right value for IsHotReloadEnabled? -->
 		<BatchMergeXaml_v0
 			ProjectFullPath="$(MSBuildProjectFullPath)"
 			Pages="@(XamlMergeInput)"
+      IsHotReloadEnabled="'$(Configuration)' == 'Debug'"
 			MergedXamlFiles="@(XamlMergeOutputFiles)" />
 		<Message Text="Merged resources XAML file was generated in $(XamlMergeOutputFile)" />
 

src/Uno.XamlMerge.Tests/Given_BatchMergeXaml.cs

@@ -28,9 +28,9 @@ public void When_Empty()
     }
 
     [TestMethod]
-    public void When_Key_TargeType_Conflict()
+    public void When_HR_Enabled()
     {
-        var task = CreateMerger();
+        var task = CreateMerger(isHotReloadEnabled: true);
 
         Assert.IsTrue(task.Execute());
 
@@ -324,7 +324,7 @@ private void ValidateOutput(BatchMergeXaml_v0 task, [CallerMemberName] string te
         }
     }
 
-    private BatchMergeXaml_v0 CreateMerger([CallerMemberName] string testName = "")
+    private BatchMergeXaml_v0 CreateMerger(bool isHotReloadEnabled = false, [CallerMemberName] string testName = "")
     {
         var basePath = GetBasePath(testName);
 
@@ -364,6 +364,7 @@ private BatchMergeXaml_v0 CreateMerger([CallerMemberName] string testName = "")
             .Distinct()
             .Select(f => new TaskItem(Path.Combine(basePath, "Output", f)))
             .ToArray();
+        task.IsHotReloadEnabled = isHotReloadEnabled;
         return task;
     }
 

src/Uno.XamlMerge.Tests/Scenarios/When_HR_Enabled/Input_Dictionary_1.xml

@@ -0,0 +1,7 @@
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
+
+    <x:String x:Key="Key1">Value1</x:String>
+</ResourceDictionary>

src/Uno.XamlMerge.Tests/Scenarios/When_HR_Enabled/Input_Dictionary_2.xml

@@ -0,0 +1,7 @@
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
+
+    <x:String x:Key="Key2">Value2</x:String>
+</ResourceDictionary>

src/Uno.XamlMerge.Tests/Scenarios/When_HR_Enabled/expected.xml

@@ -0,0 +1,9 @@
+<!-- Generating a resource dictionary that references existing dictionaries for HotReload support -->
+<!-- Proper merging of XAML files will happen if you build in Release configuration -->
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
+    <ResourceDictionary.MergedDictionaries>
+        <ResourceDictionary Source="ms-appx:///When_HR_Enabled\Input_Dictionary_1.xml" />        <ResourceDictionary Source="ms-appx:///When_HR_Enabled\Input_Dictionary_2.xml" />    </ResourceDictionary.MergedDictionaries>
+</ResourceDictionary>

src/Uno.XamlMerge.Tests/Uno.XamlMerge.Tests.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFrameworks>net6.0;net461</TargetFrameworks>
-    <LangVersion>10</LangVersion>
     <Nullable>enable</Nullable>
 
     <IsPackable>false</IsPackable>